The present invention relates to systems for locating items. In particular the invention relates to systems for locating items based on radio signals transmitted to or from transceivers provided on the items.
The use of mobile data communications systems to perform location functions for locating mobile units is described in articles entitled Enhancements to the User Location and Tracking System, by Paramvir Bahl and Venkata N. Padmanabhan, and User Location and Tracking in an In-Building Radio Network, Microsoft Technical Report MSR-TR-99-12, dated February 1999, by the same authors, both published by Microsoft Research. As described therein signal strength of signals of the wireless data communications system, such as a system using the protocol of IEEE Standard 802.11, are used for locating mobile units within an area serviced by the system. Other techniques for locating mobile units using the wireless data communications systems or other location system are possible.
In co-pending application Ser. No. 09/528,697, filed Mar. 17, 2000, which is owned by the assignee of the present application and incorporated herein by reference, there is described a system which follows the protocol of IEEE Standard 802.11, but which uses a combination of RF Ports and Cell Controllers to perform the functions of Access Points of a classical 802.11 data communications system. Lower level MAC functions are performed by the RF Ports and higher level MAC functions, including association and roaming functions, are performed by the cell controller. The term “access point” as used herein is intended to include conventional access points, such as those which follow the protocol of IEEE Standard 802.11 and perform all MAC functions, as well as RF Ports operating with cell controllers, as described in the incorporated co-pending application.
Similar systems may be implemented wherein objects are located using RFID tags applied to the items, which are interrogated by RFID readers associated with access points of an IEEE Standard 802.11 system for locating the items.
Such location systems require calibration of the radio characteristics of the area to be serviced. This can be performed by an initial radio survey of the area where a transmitter or receiver is placed at known locations in the area, and the signal characteristics for that location are determined. A location database is generated by extrapolating or interpolating the measured locations to unmeasured locations to enable location of an item at an unknown location to be determined using signal characteristics.
Industrial facilities, hospitals, universities, and the like, are rarely static, and frequent changes in the location of equipment, on a temporary or permanent basis, can change the radio signal characteristics by introducing blockage and multipath conditions. Frequently walls are erected or removed also changing the signal environment.
It is an object of the present invention to provide a system and method for determining changed signal conditions in a radio based location system.